Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
  • C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/20—Recycling

Definitions

• the invention relates to steels for use under stress at high temperature around 600 ° to 650 ° C and more particularly so-called ferritic steels with high chromium content which have a martensitic structure returned to both room temperature and temperature. on duty.
• the invention aims to apply to tubular metallurgical products such as, for example, superheater tubes, reheater tubes, manifolds or pipes for superheated or reheated steam for boilers or even tubes for chemical or petrochemicals.
• Such products are most often seamless tubes obtained after a severe hot plastic deformation operation of solid bars and are produced in very specific steels.
• austenitic stainless steel tubes of type TP321H, TP347H according to ASTM A213 have long been known for such uses. for Testing and Materials) which contain approximately 0.05% C, 18% Cr, 11% Ni and are stabilized respectively with Ti or Nb.
• These steels are very resistant to corrosion by steam due to their chromium content and have a very high resistance to creep rupture up to 700 ° C. due to their austenitic structure.
• T92 steel according to ASTM A213 specification (or P92 according to ASTM A335 specification) has a chemical composition close to T91 / P91, except that its Mo content is very reduced and that it contains 1.8% W and a micro addition of boron; the creep rupture strength ⁇ R 10 5 h 600 o C of this steel is around 120 MPa.
• T91, P91, T92, P92 steels contain 9% Cr and some of their users believe that such Cr content is insufficient to resist hot oxidation and / or corrosion by water vapor. above 600 ° C, especially at 650 ° C taking into account the metal temperature envisaged for the superheater tubes of future thermal power plants.
• This steel is supposed to be more resistant to hot oxidation than T91 or T92 because of its Cr content but it is much less resistant to creep rupture than T91 / P91 and it is difficult to weld, especially in very thick.
• the increase in the Cr content in X20 steel is compensated by a higher C content (0.20% against 0.10%) and by a moderate addition of Ni (between 0.5 and 1%).
• a C content greater than or equal to 0.20% seems undesirable for weldability.
• a significant addition of Ni offers the drawback of greatly lowering the point Ad and therefore of limiting the maximum tempering temperature of the tubes; it also seems to be detrimental to the resistance to creep rupture.
• US Pat. No. 5,069,870 discloses the addition of Cu (gamma element) at contents ranging from 0.4% to 3% on a 12% Cr steel to compensate for the increase in Cr content.
• Cu gamma element
• US Pat. No. 5,069,870 discloses the addition of Cu (gamma element) at contents ranging from 0.4% to 3% on a 12% Cr steel to compensate for the increase in Cr content.
• the addition of Cu poses forgeability problems for the manufacture of tubes of superheaters by hot rolling.
• Patent application JP 4371551 discloses an addition of Co (also gamma) between 1 and 5% (and generally more than 2%) on a steel containing 0.1% C, 8 to 13% Cr, 1 to 4% W , 0.5 to 1.5% MB, less than 0.20% Si (and in fact less than 0.11% Si) and micro-alloyed with V, Nb, N and B to obtain a very high creep rupture strength and sufficient resilience to the Charpy V test after aging.
• Co also gamma
• Patent application EP 892 079 also provides for an addition of Co at contents ranging from 0.2 to 5% but in a steel containing less than 10% Cr which does not respond to the problem set out above.
• Patent applications JP 11 061 342 and EP 867 523 also provide for an addition of Co but together with an addition of Cu for the first document and at least 1% Ni for the second document.
• the unacceptable drawbacks of such additions have been explained above.
• Patent application EP 758 025 also provides for an addition of Co, generally at very high contents; therefore, to prevent the formation of intermetallic precipitates based on Cr, Mo, Co, W, C and Fe, this document jointly provides for the addition of (Ti or Zr) and alkaline earths (Ca, Mg, Ba) or rare earths (Y, Ce, La).
• Ti or Zr however has the major drawback of forming coarse nitrides with the nitrogen of the steel and of preventing the formation of ultrafine carbonitises of V and Nb responsible for the high creep resistance.
• Patent application JP 8 187 592 also provides for an addition of Co with a particular relationship between the contents in (Mo + W) and those in (Ni + Co + Cu) but these additions and relationships are provided for to optimize the composition of materials welding filler, which are not intended to support shaping such as that during the manufacture of welded tubes (forgeability characteristics).
• Patent application JP 8 225 833 also provides for an addition of Co but relates to a heat treatment to reduce the residual austerity content and not a chemical composition; the ranges of chemical composition are therefore wide and we cannot deduce any teaching for the intended use.
• the present invention has sought to produce a steel:
• the steel according to the invention allows the manufacture of seamless tubes of small or large diameter, by various known methods of hot rolling, such as for example the Stiefel, MPM, vocational step methods , by pushing bench, by continuous rolling with reducer-puller, by Axel rolling, by planetary rolling.
• the steel considered contains by weight C 0.06 to 0.20% Si 0.10 to 1.00% Mn 0.10 to 1.00% S less than or equal to 0.010% Cr 10.00 at 13.00% Ni less than or equal to 1.00% W 1.00 to 1.80% Mo such that (W / 2 + Mo) is less than or equal to 1.50% Co 0.50 to 2.00 % V 0.15 to 0.35% Nb 0.030 to 0.150%
• B 0.0010 to 0.0100% as well as optionally at most 0.050% by weight of AI and at most 0.0100% by weight of Ca.
• the contents of constituents of the chemical composition are interconnected so that the steel after normalization heat treatment between 1050 and 1080 ° C and tempered has a martensitic structure returned free or almost free of ferrite ⁇ .
• CARBON At high temperature, especially during the hot manufacturing process of metallurgical products or during the austenitization of the final heat treatment, this element stabilizes the austenite and consequently tends to reduce the formation of ferrite ⁇ .
• the carbon is in the form of carbides or carbonitrides whose initial distribution and the evolution of this distribution over time act on the mechanical characteristics at ambient temperature and at service temperature.
• a C content of less than 0.06% makes it difficult to obtain a structure free of ferrite ⁇ and the desired creep characteristics.
• a C content greater than 0.20% is detrimental to the weldability of the steel.
• a content range of 0.10 - 0.15% is preferred.
• SILICON This element is a deoxidizing element of liquid steel which also limits the kinetics of hot oxidation by air or by water vapor, in particular according to the inventors, in synergy with the chromium content.
• Si is on the other hand an alphagene element which must be limited to avoid the formation of ferrite ⁇ and it also tends to favor the embrittling precipitations in service. Its content was therefore limited to 1.00%.
• a preferential range is from 0.20 to 0.60%.
• This element promotes deoxidation and fixes the sulfur. It also reduces the formation of ferrite ⁇ .
• a preferential range is from 0.15 to 0.50%.
• This element essentially forms sulphides which reduce cross-resilience and forgeability.
• An S content limited to 0.010% makes it possible to avoid the formation of defects during the hot drilling of billets during the process of manufacturing seamless tubes.
• a content as low as possible, for example less than or equal to 0.005%, or even 0.003% is preferred.
• This element is both dissolved in the steel matrix and precipitated in the form of carbides.
• a minimum Cr content of 10% and preferably 11% is necessary for resistance to hot oxidation. Given the alphagene nature of chromium, a content greater than 13% makes it difficult to avoid the presence of ite ferrite.
• NICKEL It promotes resilience and prevents the formation of ferrite ⁇ , but greatly reduces the temperature Ad and therefore decreases the maximum tempering temperature of the steel.
• a content greater than 1% is therefore not desirable, especially since nickel tends to reduce the resistance to creep rupture. It will be preferable to limit the maximum Ni content to 0.50%.
• This element which is both dissolved and precipitated in the form of carbides and intermetallic phases is fundamental for the creep resistance at 600 ° C and above, hence a minimum content of 1.00%.
• the inventors have found it unwise to increase the W content beyond 1.80%.
• This element has an effect similar to tungsten even if it appears less effective for resistance to creep.
• the molybdenum content is preferably less than or equal to 0.50%.
• This element stabilizes the austenite and therefore allows to tolerate more than 10% of Cr and it also improves the creep resistance properties; a minimum content of
• this element contributes to entering into embrittling intermetallic compounds capable of precipitating at service temperature and it is also very expensive.
• this element has mainly been used at contents higher than 2% in materials for use considered at high temperature to improve their resistance to rupture by creep.
• the inventors of the present invention have surprisingly found that the range of cobalt content from 0.50 to 2.00% and preferably from 1.00 to 1.50% allows to meet the objectives set for this steel and in particular an optimal compromise of different possibly contradictory characteristics (for example, resistance to oxidation, resistance to creep and forgeability), with a relatively simple metallurgy and a limited manufacturing cost of metallurgical products. This is not the case with steels containing more than 2% Co which have so far been highlighted.
• This element forms very fine and stable nitrides and carbonitrides and therefore very important for resistance to rupture by creep.
• a content of less than 0.15% is insufficient to obtain the desired result.
• a content higher than 0.35% is harmful with regard to the risk of the appearance of ferrite ⁇ .
• a preferential range is from 0.20 to 0.30%.
• this element forms stable carbonitrides and its addition strengthens the stability of the vanadium compounds.
• Nb content of less than 0.030% is insufficient to do this.
• An Nb content greater than 0.15% is unfavorable, the carbonitrides of Nb then being able to have an excessive size and reduce the creep resistance.
• a preferred range is from 0.050 to 0.100%.
• NITROGEN This gamma element reduces the appearance of ferrite ⁇ .
• a minimum nitrogen content of 0.030% is therefore required.
• a nitrogen content greater than 0.120% leads on the steels considered to blowing on ingots, billets or slabs and consequently to defects on metallurgical products. The same risk exists when welding the implementation of these products.
• a nitrogen content range of 0.040% to 0.100% is preferred.
• This element helps stabilize carbides when added to more than
• a content greater than 0.0100% can, on the other hand, greatly reduce the burning temperature of products, particularly raw products, and therefore appears unfavorable.
• This element is not necessary in itself to obtain the desired metallurgical characteristics and it is here considered as a residual process; its addition is therefore optional.
• aluminum can be added if necessary to obtain a final content of up to 0.050%.
• a Ca or Mg content of less than 0.0010% results from exchanges between liquid steel and production slag containing lime or magnesia in a highly deoxidized medium: these are therefore inevitable production residues.
• Calcium can however optionally be added at contents a little higher than 0.0010% to improve the flowability and / or to control the shape of the oxides and sulfides.
• a Ca content greater than 0.0100% indicates an oxygen-rich and therefore dirty steel and is therefore unfavorable.
• the steel according to the invention contains as other elements only impurities such as, for example, phosphorus or oxygen and residuals originating in particular from scrap iron put in to make steel or coming from exchanges with slag or refractories or necessary for the production and casting processes.
• impurities such as, for example, phosphorus or oxygen and residuals originating in particular from scrap iron put in to make steel or coming from exchanges with slag or refractories or necessary for the production and casting processes.
• Ti or Zr contents of less than 0.010% thus result from the scrap iron put in and not from voluntary addition; such low contents have no significant effect on the steel for the use considered.
• the copper content (also resulting from the scrap iron placed in the oven and not from voluntary addition) remains less than 0.25% and optionally less than 0.10%. Levels higher than these contents may prohibit certain hot rolling processes of seamless tubes and force the use of more expensive glass spinning processes.
• RELATIONSHIP CHEMICAL COMPOSITION AND FERRITE CONTENT ⁇ Steelmakers know how to balance the chemical composition of a steel containing about 12% Cr by targeting an absence or almost absence of ferrite ⁇ after heat treatment from a relationship between the contents of elements of chemical composition.
• structure almost free of ferrite ⁇ is meant a structure containing not more than 2% of ferrite ⁇ and preferably not more than 1% of ferrite ⁇ (measured with an absolute precision of ⁇ 1%).
• FIG. 1 represents a diagram of ferrite content ⁇ - equivalent chromium content for different samples of steels containing 8 to 13% Cr treated thermally.
• FIG. 2 represents a diagram of the results of forgeability tests on the steel F according to the invention compared to other steels.
• FIG. 3 represents, for the same steel F compared to other steels, a diagram of the results of hot tensile tests, FIG. 3 a) being relative to the elastic limit and FIG. 3 b) to the resistance to breaking.
• Figure 4 represents for the same steel F compared to other steels a transition curve of the Charpy V resilience.
• FIG. 5 represents for the same steel F compared to other steels a curve of results of rupture tests by creep under constant unit load.
• FIG. 6 represents for the same steel F compared to other steels a master curve of results of creep rupture tests under different loads as a function of the Larson-Miller parameter.
• a 100 kg laboratory steel casting according to the invention was produced under vacuum (reference F).
• the Cr equ parameter comes from the work of Patriarca et al. (Nuclear Technology, 28 (1976) p. 516).
• This FIG. 1 made it possible to determine the analytical aim of the casting F within the ranges of contents of the elements of the chemical composition defined by claim 1. We thus aimed to obtain a Cr equ content less than or equal to 10, 5% and if possible less than or equal to 10.0% so as to seek to obtain an almost ferrite-free content ⁇ (less than 2% and preferably less than 1%) after heat treatment.
• Table 1 chemical composition (in% by weight).
• Table 1 provides the chemical composition of this flow F and the average chemical composition of shades known from the state of the art (% by weight) as well as the corresponding value of the parameter Cr equ .
• This flow F does not contain any addition of Ca and its Al content is less than 0.010% (residual Al and Ca from production).
• the ingots obtained were reheated to 1250 ° C. and then hot rolled into 20 mm thick sheet which was then subjected to stress relieving.
• a metallographic sample taken in the long direction from this sheet was first examined with an optical microscope after metallographic attack with Villela reagent.
• Samples were then taken in the transverse direction to carry out forgeability tests by hot traction at an average speed of deformation of 1 s ' 1 .
• the forgeability tests were carried out comparatively on these samples of casting F and on samples from a laminated bar of diameter 310 mm in P91 steel and of a laminated bar of diameter 230 mm in P92 steel.
• FIG. 2 shows the results of necking at break which have been obtained.
• necking at break remains greater than 70% from 1200 ° to 1320 ° C and is comparable to that of a P92.
• Table 2 evolution of the ferrite content ⁇ at high temperature.
• the ferrite content values ⁇ obtained are comparable to those measured under the same conditions on comparative steels P91, P92.
• the ferrite content ⁇ is less than 15% up to 1250 ° C and less than 20% up to 1280 ° C.
• the burn temperature is more than 1320 ° C.
• Seamless tubes should therefore be able to be produced by many productive hot rolling processes, therefore able to be produced at relatively low cost. It is not the same for tubes in austenitic grades or in shades containing 12% Cr and 1% Cu which require, at least for tubes of small diameter of the kind of superheater tubes, to be produced so less productive by glass spinning.
• Table 3 shows the results obtained compared to typical results on known steels.
• the temperature Ad of 830 ° C for steel F is comparable to that of P91 and P92 and significantly higher than that of P122 in Copper which does not allow an annealing temperature above 780 ° C.
• An income at 800 ° C on the contrary is quite possible on steel F according to the invention.
• microstructure and the hardness were measured after normalization heat treatment for 20 min at 1060 ° C (N1 treatment) or 1080 ° C (N2 treatment); the results are shown in Table 4.
• Table 4 results after normalization heat treatment. The microstructure and hardness were also measured after heat treatment for normalization N1 and returned for 1 hour at 780 ° C (T1), 30 min at 800 °
• Table 5 results after normalization and income.
• the Charpy V resilience characteristics were then measured in the long direction at test temperatures ranging from -60 ° to + 40 ° C after heat treatments N1 + T1, N1 + T2 or N1 + T3.
• the creep rupture characteristics were then determined by different tests at different temperatures under constant unit load (140 and 120 MPa) compared with the steel F of the present invention (heat treatments N1 + T2 or N2 + T2) and on a tube in P92.
• FIG. 5 The results of the breaking test duration under 120 MPa are illustrated in FIG. 5 as a function of the parameter 1000 / T (in ° K -1 ) conventionally used for this type of grade.
• the temperatures have been chosen so that the maximum duration of the test is close to 4000 h.
• FIG. 5 allows the temperature corresponding to a test duration of 10 5 h to be extrapolated for the unit test load. It can be seen that this temperature is for steel F at least equal to, if not higher than, that of steel P92.
• Hot oxidation tests in water vapor have been undertaken on product F in the N1 + T2 state at 600 ° and 650 ° C for periods of up to 5000 hours 10 compared to different steels for use in high temperature according to ASTM A213 or according to DIN 17175:
• Table 7 results of hot oxidation tests after 1344 h.
• the corrosion rates on grade F according to the invention are extremely low, lower even than on a sample of austenitic steel 347H containing 18% Cr and almost as low as on a sample of steel 347 GF (also austenitic at 18% Cr) which is a benchmark for resistance to hot oxidation.
• the steel according to the invention therefore makes it possible to produce boilers having a vapor temperature greater than 600 ° C. totally made of ferritic steels, including for the hottest parts.
• grade F according to the invention is perfectly satisfied with sulfur contents less than or equal to 0.005%, or even even less than or equal to 0.003%, and does not require the addition of rare earths and / or alkalino- earthy delicate to implement.
• Table 9 Chemical composition (in% by weight) of the 53058 steel casting according to the invention.
• Ingots were forged into solid bars with a diameter of 180 mm which were transformed into seamless tubes with an outside diameter of 60.3 mm and a thickness of 8.8 mm by the continuous rolling process on a selected mandrel with reduction in diameter on a rolling mill. reducer-shooter.
• Table 10 shows the results of ambient tensile tests on tubes treated by standardization at 1060 ° C and tempered for 2 hours at 780 ° C.
• Table 11 shows the results of Charpy V impact tests on tubes having undergone the same heat treatment as for the tensile tests.
• Table 10 results of ambient tensile tests on steel tubes according to the invention.
• Table 11 Charpy V impact test results on steel tube according to the invention.

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